Ruthenium catalysed hydrogenation of dimethyl oxalate to ethylene glycol
Herman T. Teunissen and Cornelis J. Elsevier*
Anorganisch Chemisch Laboratorium, J. H. van’t Hoff Research Instituut, Universiteit van Amsterdam, Nieuwe Achtergracht 166,
NL-1018 WV Amsterdam, The Netherlands
Dimethyl oxalate is efficiently hydrogenated to ethylene
glycol under mild conditions [p(H ) 70 bar; 100 °C] using a
ruthenium catalyst based on Ru(acac) and
MeC(CH PPh
the activity of the ruthenium–phosphine catalysts in the
hydrogenation of DMO (expressed as turnover frequency)
2
3
increases in the order P(C
6
H
11
)
3
< Ph
2
PC
2
H
4
PPh
PPh
2
< PPh
3
<
2
2
)
3
.
PhP(C
MeC(CH
MeC(CH
2
H
4
2
2
PPh
PPh
PPh
)
2 2
)
2 3
)
2 3
≈
[CH P(Ph)C
2
2
H
4
2
]
2
<<
.
The extraordinary effect of the
ligand on the catalytic activity is evident
5
The homogeneous hydrogenation of esters to alcohols is a
difficult process and only a few papers dealing with this subject
have appeared.1 Anionic ruthenium hydride catalysts have
from the formation of EGL in high yield which was not
observed in any previous experiment. (Note that the hydro-
genation of MGL to EGL is not as activated by an electron-
withdrawing substituent as the hydrogenation of DMO to MGL,
–3
1
been described by Grey et al. and neutral ruthenium catalysts
were reported by Matteoli et al.2 In general, drastic conditions
are required for the efficient conversion of an ester to the
corresponding alcohol, unless the ester is activated by electron-
withdrawing substituents. Thus, the hydrogenation of methyl
,3
vide supra.) Our catalytic system consisting of Ru(acac)
3
and
the tridentate ligand MeC(CH PPh (entry 11) is considerably
2
2 3
)
better with respect to the selectivity of EGL formation and the
glycolate (MGL) to ethylene glycol [EGL, eqn. (1)] requires
turnover frequency when compared with the catalytic properties
of the best known system so far, i.e. Ru(CO) (AcO) (PBu ) .
2 2 3 2
2
2
2
drastic conditions [p(H ) 200 bar; 180 °C] while dimethyl
oxalate (DMO) is relatively easily reduced to MGL [eqn.(2)].
Furthermore, our system displays catalytic activity under
relatively mild conditions. The catalytic properties of the
ruthenium catalyst derived from MeC(CH PPh ) indicate that
2 2 3
a ruthenium complex with a fac coordinating ligand is essential
for high catalytic activity. This conjecture is supported by the
MeO
MeO
2
CCH
CCO
2
OH + 2 H
Me + 2 H
2
2
? HOCH
2
CH
2
OH + MeOH (1)
2
2
? MeO CCH
2
2
OH + MeOH (2)
We report a ruthenium based homogeneous catalyst which is
able to homogeneously catalyse the hydrogenation of DMO to
EGL under substantially milder conditions.
relatively low catalytic activity of the PhP(C
[CH P(Ph)C PPh derived systems, in which the ligands
can coordinate either in a fac or mer fashion to ruthenium.
The Ru(acac) –MeC(CH PPh system was explored in
2 4 2 2
H PPh ) and
2
H
2 4
2 2
]
Exploratory experiments were based on the Ru(acac)
3
3
2
2 3
)
system, described by Hara and Wada for the hydrogenation of
anhydrides to lactones. These experiments revealed that an
active catalyst can be generated in situ from Ru(acac) and a
3
more detail with respect to the influence of the hydrogen
pressure, the substrate to catalyst ratio, and the role of zinc.
From Table 2 it is clear that the turnover number increases from
160 (entry 1) to 642 (entry 3) when the Ru:DMO ratio
decreases from 1.19 to 0.18%. As is seen from entries 8 and 9,
decreasing the Ru:DMO ratio from 0.20 to 0.15 leads to
significantly lower catalytic activity; the turnover frequency
4
donor ligand in MeOH in the presence of zinc. Zinc was added
to initiate a fast reduction of the acetylacetonate complex. The
influence of ligands on the catalytic activity of this system was
explored in detail and is shown in Table 1.†
From Table 1 it is evident that ruthenium catalysts with
phosphine ligands (except tricyclohexylphosphine, entry 7)
show a higher activity than with the nitrogen or arsenic
compounds (entries 3–6). The applied phosphine ligands
induced remarkable differences in terms of catalytic activity as
a function of their coordination properties. More specifically,
2
1
decreases from 53.5 to 15.5 h
As expected, the H pressure has a significant influence on
the turnover number and turnover frequencies. Decreasing the
pressure from 70 to 20 bar (entries 3–5) leads to a decrease
.
2
H
2
of the turnover frequency by a factor 3.6. Comparing the
experiments in entries 3 and 6 (and 7 and 8), the influence of
Table 1 Influence of the ligand in the ruthenium catalysed hydrogenation of DMO to MGL and ethylene glycol (EGL)a
Conver-
sion
Select-
Yield ivity
MGL MGL EGL
Select-
Yield ivity
Turn-
over
Turn-
over
number
DMO/
Entry mmol
Ru:DMO
(%)
Ligand:Ru Zn:DMO DMO
EGL
(%)
freq./
2
1
Ligand
Noneb
(%)
(%)
(%)
(%)
(%)
(%)
h
1
2
3
4
5
6
7
8
9
0
1
2
0.96
0.99
1.41
1.04
0.89
9.25
0.89
0.88
1.14
0.96
1.77
11.00
1.64
1.98
1.19
1.96
1.74
0.21
2.18
1.82
1.75
2.38
1.19
1.34
0.00
5.88
8.91
6.39
1.79
2.32
4.63
2.94
1.68
1.02
1.37
4.00
0.27
1.32
0.57
0.33
0.38
0.06
0.25
0.42
0.36
0.33
0.26
0.00
18
73
1
20
11
14
7
18
76
91
100
100
2
36
0
0
0
1
1
11
67
85
1
10
49
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
1
18.1
0
0
0
2.9
0.5
5.9
38.2
35.7
160
136
0
0.9
0
0
0
0.2
0
0.4
2.5
2.2
PPh
AsPh
3
3
1,10-Phenanthroline
2,2A:6A,2B-Terpyridine
Pyrazolyl ligandc
4
P(C
Ph PC
PhP(C
[CH P(Ph)C
MeC(CH PPh
Ru(CO) (OAc)
6
H
11
)
H
3
18
60
89
93
1
2
2
4
PPh
2
2
H
4
PPh
2
)
2
1
1
1
2
2
H
4
PPh
2
]
2
0
95
82
0
95
82
2
2
)
3
10
0.9
d
2
2
(PBu
3
)
2
18
18
a
) = 70 bar, 16 h, MeOH (12 ml), used as received. b Reaction time 41 h. c Tris(3,5-
) = 200 bar, 144 h.
3 2
Conditions (unless otherwise indicated): Ru(acac) , 100 °C, p(H
d
dimethylpyrazol-1-yl)borohydride. From ref. 2; reaction at 180 °C in MeOH, p(H
2
Chem. Commun., 1997
667